The present specification generally relates to optical data storage. More particularly, the present specification describes an electro-optical system for data storage and retrieval in a near-field recording configuration.
Data storage is an important aspect of today""s information technology. A great deal of effort has been made by the storage industry to increase the areal data density of a storage medium in order to meet the ever increasing demand for higher capacity storage devices.
Magnetic storage devices such as fixed or removable magnetic disks and tapes are widely-used conventional storage devices. The state-of-art conventional magnetic hard drive systems can achieve extremely high linear bit densities, especially with the new MR and GMR magnetic heads. For example, the areal density of many hard disk drives is on the order of magnitude of about one gigabit per square inch. One limitation in increasing areal data density in a magnetic device is the particle size or the characteristic dimension of a typical magnetic domain of the magnetic recording materials. Other limitations include the width of the magnetic read/write head and the limitations of mechanical tracking. Therefore, these hard drives are typically limited to less than 10,000 tracks per inch.
Optical storage devices are emerging as an alternative technology to the conventional magnetic technology because of their potential for high density data storage. The areal density of an optical storage device, in principle, is only limited by the diffraction limit of an illuminating optical beam for reading or writing. One type of commercial optical storage technology is based on magneto-optical materials. These materials can currently produce an areal data density of about one giga bit per square inch.
One well-known approach to increase the areal data density in an optical storage system is using smaller beam size. Due to the diffraction limit, this may be achieved by using a light source with shorter wavelengths such as those toward the blue end of the spectrum. For example, one application for the industrial development of compact blue lasers is aimed at the optical storage. Alternatively, one may increase the numerical aperture of the objective lens in the system to focus a beam at a given wavelength to a smaller spot within the diffraction limit.
FIG. 1 shows a block diagram of a typical rewritable optical storage system or drive 100 based on magneto-optic recording in the prior art. Several servo mechanisms are needed to keep the laser beam in focus and tracking on the disk, for example, an objective lens actuator 114, a master-slave tracking servo control 130, and a focusing servo control 120. In particular, the objective lens in the prior-art system 100 is servo controlled for focusing and tracking the beam onto the storage medium layer(s) at a desired location. This type of conventional optics system is usually limited to numerical apertures of the objective lens of less than 1.0, and typically in a range about 0.55 to 0.60. Since the areal density of the data stored on the medium is directly proportional to the square of the numerical aperture, the limited numerical apertures of a conventional optical drive can significantly restrict a substantial increase in the data density.
The present disclosure includes an electro-optical storage system with an areal data density that is higher than that of the prior-art storage systems such as state-of-art magnetic hard disk drives and various optical drives. One embodiment of the systems of the present invention comprises a read/write head and a head positioning system, an optics module including beam relay optics and signal detectors, an optical medium and a corresponding medium driving unit, and an electronic control system.
The read/write head is preferably a xe2x80x9cflyingxe2x80x9d head which is suspended over the optical medium by an air-bearing surface in a near-field recording configuration wherein the spacing between an exit facet of the flying head and a recording layer in the medium is a fraction of the wavelength. An optical read/write beam exiting the near-field lens is then coupled to the optical medium by evanescent waves. The flying head includes a near-field lens with a high index of refraction and usually has a numerical aperture greater than unity under the preferred near-field condition. A focused beam with a spot size smaller than that obtainable from a conventional optical system is thus achieved at least in part due to the use of a high index solid immersion lens (xe2x80x9cSILxe2x80x9d) as the near-field lens.
One aspect of the invention is the automatic optimization and maintenance of focus under the preferred near-field condition. This is accomplished, at least in part, by the use of the air-bearing surface to suspend the flying head over the surface of the optical medium by a fraction of a wavelength at a prespecified height. Therefore, a conventional focusing servo system may not be required.
According to one embodiment, a solid immersion lens is used as the near-field lens with respect to an objective lens at a desired distance. A SIL cap lens that is part of a sphere may be laminated to a transparent base plate with an optical UV epoxy layer. A spacer having a void area that is larger than the SIL cap lens may be adhered to the base plate with the optical UV epoxy layer in a way so that the SIL cap lens is enclosed in the void area of the spacer. The thickness of the spacer is preferably at least the height of the SIL cap lens. The objective lens is then fixed to the spacer with an epoxy. The desired distance between the objective lens and the SIL cap lens may be determined in an alignment process by maximizing an optical feedback signal from an exit facet of the SIL cap lens. A transparent mesa may be formed on the base plate as a part of the near-field lens for coupling light between the flying head and the optical medium. The SIL cap lens and the base are preferably made of materials that have a similar index of refraction, including but not limited to cubic Zirconia, Schott glass (LaSF35), Hoya glass (TaFd43), Cleartran, Zinc Selenide, Gallium Phosphide and others. In one implementation, the index mismatch at the operating wavelength should be less than about 2% for optimal performance.
The optics module may be a fixed optics module, i.e., the relative positions of different optical elements within are fixed at predetermined distances. In one embodiment, the fixed optics module includes a light source, a collimator lens, an anamorphic prism, a front facet monitor, a polarization rotator, a data/servo detector, a relay lens, a galvanometer (xe2x80x9cgalvoxe2x80x9d) mirror, and a folding mirror for guiding a read/write beam to the flying head. The orientation of the galvo mirror is controlled to provide a fine positioning mechanism for precisely positioning the read/write beam to a desired point on the optical medium. In accordance with one embodiment, the galvanometer may have a compact and improved Winchester flexure with two load points on a rigid stiffener to define a single axis of rotation that is close to the reflecting surface of the galvo mirror. One or more capacitive position sensors may be implemented in the galvanometer for position monitoring and controlling.
A passive thermal compensation scheme may be implemented in the fixed optics module to maintain an optimal focus. The thermal and mechanical properties of optics mounting devices supporting the optical train of the disk drive are carefully chosen with respect to one another to minimize the overall thermal variation of the optical train over a certain temperature range. In addition, various mounting techniques can be used so that thermal expansion of different parts of a device may cancel one another. Furthermore, optical component materials can be selected to minimize the overall thermal effect.
A rotary actuator may be used as a coarse positioning means for the optical disk drive although other positioning devices may also be used. The fixed optics module and the flying head are attached to an actuator arm of the rotary actuator. Hence, any user data sector on the optical medium may be addressed with a read/write beam by adjusting the rotary actuator and turning the galvo mirror.
The optical medium can be writable/erasable materials (i.e., write-many-read-many), write-once-read-many materials, and read-only materials. Preferred writable/erasable materials are the magneto-optic type, including but not limited to, rare earth-transition metal compounds such as TbFeCo. According to one embodiment, a multilayer structure with at least one magneto-optic recording layer has a reversed layer construction compared to a conventional multilayer magneto-optic medium. A first top dielectric layer, a magneto-optic recording layer, a second dielectric layer, and a reflective substrate may be formed in sequence. This unconventional multilayer construction is for the first surface recording under the preferred near-field condition wherein the distance between the recording layer and the flying head is a fraction of the wavelength.
According to the invention, the optical medium may also have a plurality of recording layers in a multilayer construction. In one embodiment, an optical flying head having a hemispherical SIL lens may be operated within the focusing tolerance range to address any one recording layer, thus effectively increasing the areal data density of the medium. In another embodiment, an optical flying head having a hemispherical SIL lens may be operated in a hemispherical regime to address a first top recording layer and operated in a super-hemispherical regime to address a second recording layer in an optical medium. In this embodiment, the top recording layer forms part of the SIL. Switching between the two operating regimes may be accomplished by, for example, adjusting the position of the relay lens.
The multilayer structure of the medium can be configured for optimized signal detection by minimizing variations in the signal reflectivity and in compensating for variations in the flight height of the flying head. In one embodiment, a multilayer structure sequentially comprises a first dielectric layer with a high refractive index (e.g., SiN), a second dielectric layer with a low refractive index (e.g., SiOx), a magneto-optic recording layer, a third dielectric layer of a high refractive index, a reflective layer (e.g., Al), and a substrate which may be made of plastics, glasses or metals (e.g., Al).
A headerless magneto-optic disk format may be used with advantages in accordance with an embodiment. In one preferred wedge format, the disk includes a plurality of wedges intersecting all the tracks by xe2x80x9cspokexe2x80x9d type wedge ID fields and the track numbers and wedge numbers are written along the radial lines at a fixed frequency on a magneto-optic disk and are independent of radial locations. Each wedge comprises a small ID field and a data field for storing the actual user data. The wedge ID field may be further partitioned into multiple sub fields including three gap fields of different sizes, one sub field for an automatic-gain-control field, one sub field for the sector/index address mark, one sub field for the track number, one sub field for the wedge number, and one sub field for the cyclical redundancy code to verify error-free readout of the track number and the wedge number.
One aspect of the invention is a disk cartridge with a self-cleaning mechanism based on electrostatic forces to remove contaminant particles from the disk surface. In one embodiment of the invention, a cleaning element is mounted on an interior surface of the cartridge facing a recording surface. The cleaning element includes a base with one end of a lightweight flexible tape fixed thereon. The tape may be made of a flexible electret polymer material capable of holding a high electric charge for a long period of time. The cleaning tape may also be self-charging by, for example, constructing the tape with two materials that, when rubbed together, generate equal and opposite charges. The tape may be made as strands or fibers to increase the rubbing surface area.
A disk drive may implement a disk cartridge carrier to minimize contamination by keeping a disk out of reach of a user at all times. A special box-like carrier is used to xe2x80x9clockxe2x80x9d a cartridge therein when the disk is not in use. The carrier includes a door for loading and unloading the cartridge. A special docking system is implemented in the disk drive. This system keeps a cartridge from being in direct contact with any objects other than the carrier and the disk drive. In loading a disk, the carrier is temporarily docked to the drive. The carrier door is then opened and the cartridge is automatically removed from the carrier and transferred into the disk drive. At this time, the empty carrier can be removed from the disk drive. In unloading a disk, an empty carrier is temporarily docked to the drive. The docking system automatically transfers the cartridge from the disk drive to the carrier. The cartridge enclosed in the carrier is then removed from the disk drive. The carrier door remains closed and locked if the carrier is not docked to the disk drive.
A near-field lens in accordance with the invention may be implemented in a mastering station to reduce the track pitch since a numerical aperture higher than conventional lenses can be achieved with either a solid immersion lens or a graded index lens. A flying head with the solid immersion lens or a graded index lens is suspended over a photoresist layer coated on a glass mastering blank disk during a photoresist exposure.
The various optical storage systems in accordance with the invention can have many advantages over the conventional systems. For example, the use of a flying optical head to maintain focus through an air-bearing action of a slider mechanism can eliminate the focus servo electronics and lens actuator system in the conventional systems. The near field recording mechanism in accordance with the invention allows the numerical aperture of the focusing optics to be much greater than 1.0 and typically more than 2.0. This can be used to increase the data areal density by an order-of-magnitude over any optical storage system today despite the laser wavelength used.
In addition, one implementation of an optical storage system described herein may use a two-stage tracking system through the near-field optical head, allowing the use of a high bandwidth tracking galvanometer mirror. This type of optical tracking can be used to achieve greater than 100,000 tracks per inch, thereby providing much greater areal storage density than either conventional optical or magnetic storage systems in use today.